oil pressure

LITEDAZE · Post by **LITEDAZE** » Sun Oct 07, 2007 1:55 pm

i can't seem to get an answer for this:

at first i thought my oil pickup hose fell off the bottom of the pump, because i had no oil pressure....until i overfilled it with oil to see if the pump would work if it was touching oil. by doing this, i got my oil pressure back, so i knew the pump was ok.

after driving for a while at a slow speed to test it out, i noticed my oil pressure dropping. i figured it was because it was overfilled and the oil was foaming, which would cause the pump to suck foam and drop the pressure.

i then decided to give it one last test: take the oil out, change the filter, and put the right amount of clean oil in. now i'm getting pressure....fine right? nope.....my pressure still drops after about 15 minutes...sometimes i have to rev it a little bit when stopped at a light so that it stays at a healthy level.

so it can't be the pickup hose, and the pump IS working, but something ain't right.
removing the oil pan from a Northstar engine involves alot of hours of work, which shouldn't be done in an open driveway due to safety issues....

since we have a couple blown N*s, the idea was to cut into the pan of one of the spare engines to get it right, then cut into mine, and either re-fasten the pickup hose or replace the pump. we would then have either tacked the plate back on or made a gasket and bolt a plate back onto the bottom of the pan.

but now i'm not sure what needs to be fixed....anyone else ever have this stupid problem?

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Oct 07, 2007 2:45 pm

What weight oil are you using?

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Oct 07, 2007 2:47 pm

Also, you using a known good mechanical gauge? Also how low is low? 10 psi? 5 psi?

What it at idle hot?

LITEDAZE · Post by **LITEDAZE** » Sun Oct 07, 2007 2:54 pm

using 5w30, i was told to use this, should i consider an alternative?
and yes the gauge is good.

when i start driving it stays at 60-80, a few minutes brings it to 40, about 10 more i'm at 20....at idle hot i have to rev it to make it rise to 20 because it looks like it will go to 5 or under.

Shaun41178(2) · Post by **Shaun41178(2)** » Sun Oct 07, 2007 3:02 pm

Is it a high mile motor? Seems as the oil heats up, it thins out dropping the oil pressure.

Bearing clearences might be going out of spec.

Try a thicker oil. Like a 20/50 Thats what I run in all my engines, even newer ones.

LITEDAZE · Post by **LITEDAZE** » Sun Oct 07, 2007 3:25 pm

it is a higher mile motor, but wow, i cant believe it drops that low. since this is the case, is going up to 20 just gonna buy me another hour?

The Dark Side of Will · Sun Oct 07, 2007 4:47 pm

The oil pressure drops because the oil's heating up. Once the oil temp stabilizes, pressure won't drop anymore.

Do you have an oil cooler of any kind? How are you plumbing in your check gauge? You're not relying on the Fiero gauge are you?

Engines don't need a lot of oil pressure at idle. I've never seen the "official" numbers for a Northstar, but a Chevy only needs 10 psi or so to idle.

Blue Shift · Post by **Blue Shift** » Sun Oct 07, 2007 7:14 pm

Speaking of Chevy, my blown boat engine (dried the pump pickup out doing donuts with the girl onboard, oops) was knocking noticeably as we limped back to the dock, but when I started it up the next morning, it sounded fine till the oil got warm. I'd say that on the condition that the oil pressure is indeed unacceptable, that it's likely the bearing clearances.

whipped · Post by **whipped** » Sun Oct 07, 2007 9:03 pm

What year is your N*?

There were some of the earlier models that had a problem with the oil pump check ball. It could stick, giving you 0 oil pressure unless you overfilled it.

There is a fix:

http://www.cadillacforums.com/forums/no ... gency.html

There's also only 1 bolt holding the pickup on... lose that and it falls out.

LITEDAZE · Post by **LITEDAZE** » Mon Oct 08, 2007 4:00 am

it's a 93 N*...my pick up hose has to be on because it has the right amount of oil in it and i do get pressure, it just fluctuates more than i've ever seen

thanks for the link, i'm going to try that out.

Pyrthian · Post by **Pyrthian** » Mon Oct 08, 2007 10:26 am

The Dark Side of Will wrote:The oil pressure drops because the oil's heating up. Once the oil temp stabilizes, pressure won't drop anymore.

Do you have an oil cooler of any kind? How are you plumbing in your check gauge? You're not relying on the Fiero gauge are you?

Engines don't need a lot of oil pressure at idle. I've never seen the "official" numbers for a Northstar, but a Chevy only needs 10 psi or so to idle.

yup. as the oil heats, it gets thinner.

LITEDAZE · Post by **LITEDAZE** » Mon Oct 08, 2007 3:28 pm

Pyrthian wrote:
yup. as the oil heats, it gets thinner.

yes, this is common knowledge, but i figured something has to be wrong if the pressure drops dangerously low in no-time...we're talking about a 75psi change here.

The Dark Side of Will · Mon Oct 08, 2007 4:21 pm

Significant differences in cold and hot oil pressure are not uncommon. The Fiero pressure gauge has much less damping than most, so you see these fluctuations practically in real time.

Does the oil pressure light come on if you let the pressure go as low as it's going to go?

How do you know your gauge is accurate?

Are you using the stock sensor location in the oil filter adapter? What sensor are you using?

LITEDAZE · Post by **LITEDAZE** » Mon Oct 08, 2007 4:55 pm

i'm using a sunpro gauge cluster that was correectly installed. the light doesn't come on in the fiero display, but i don't know if it's still connected or not.

i can feel the difference though, and hear it when it gets at it's lowest....it doesn't knock, it just sounds rougher than it does when it's above 20. when the pressure gets low, the throttle becomes less responsive.

Pyrthian · Post by **Pyrthian** » Tue Oct 09, 2007 1:58 pm

LITEDAZE wrote:
Pyrthian wrote:
yup. as the oil heats, it gets thinner.
yes, this is common knowledge, but i figured something has to be wrong if the pressure drops dangerously low in no-time...we're talking about a 75psi change here.

yes - but the degree is not. on high pressure oil pump cars - starting in the winter can in fact BURST your oil filter.

Mach10 · Post by **Mach10** » Tue Oct 09, 2007 2:20 pm

Thoughts:

1) Oil heats up MUCH faster than the rest of your engine. It does this for a couple of reasons. First, It has a much lower heat capacity (heats up faster, cools off quicker because it doesn't have the "inertia" that water does. Will can explain this better--it has to do with density, as well as the actual makeup of the various components of oil).
Second, Oil heats up faster because it isn't soaking up exhaust heat (not at first, anyway) like water, but rather heats up directly due to physical stress (getting repeatedly squished into very narrow, extremely high pressure spots tends to warm you up.). This is important to note because the engine block is COOLING the oil, since it's "hot" (at the bearings, anyway) almost immediately. Again, I'm probably not explaining this well.

2) "fluctuating" oil pressure can mean a number of things; Usually it means a bad pressure sender, since the oil pump is a pretty simple rotary device. Unless the idle is fluctuating too, the flow will tend to be reasonably smooth, unless there's something else mechanically wrong. Ideas:
-loose pickup tube--sucking in air bubbles
-defective/leaky bypass on either the filter or pump itself
-badly worn oil pump internals causing cavitation, sucking air, or other nastiness.

3) Low pressure at idle isn't necesarily bad; I've heard from builders that at unloaded idle, it'll do fine with any kind of oil flow... But that the "golden number" is around 10psi / 1000rpm. Given that the Fiero "oil" lamp fires at around what, 5-6psi, you're probably ok.

Things like oil coolers, remote filters... Anything that increases the size of the pressurized oil system WILL reduce idle oil pressure. Especially at idle when the oil pump isn't operating anywhere near "ideal," and even more so considering it's a well worn engine.

4) That the engine gets a little noisy at hot idle isn't nothing unusual for a DOHC hydraulic tappet motor. If it's literally killing performance, then I'd suggest you have some other problems; possible that the hydraulic tappets are bleeding off and your valves aren't opening properly? Although this is usually accompanied with a fairly solid (loud) lifter tapping

In that case, you've also got other problems.